SwaleageAugust 3rd, 2015. Post by Quinta do Vale
It’s been a long time since this blog was last updated. Those keeping up with us on Facebook will have some inkling of what’s been going on at the quinta in the meantime, but I’ve failed dismally at getting to the more detailed documentation of it all. Mostly a case of too busy doing the doing to be reporting the doing …
Following the successful implementation of a swale system on the bottom terrace last year, this last Spring I put in a similar system on the terrace above it. It’s a narrow terrace with very similar problems to the one below it – soil so dry in summer it barely supported a few fruit trees (which consequently dropped most of their fruit before it got anywhere near ripe) amongst grasses and wildflowers which would be dry and dead by July. In summer, the soil turned to dust in your hand and blew away.
A simple solution to the terrace’s immediate problems would have been to clear the grasses from around the trunks of the fruit trees, mulch heavily and install drip line irrigation to the trees, but the underlying aim of much of the work here has been to improve the water-holding capacity, life and health of the soil to a point where irrigation becomes only minimally necessary.
Grass-clearing and mulching around the trees had already been done. The next step was to address the soil.
In common with most of the terracing on the quinta, the flatter land that’s been created for growing is still far from being flat. Gradients have been reduced from between 30-45°, but average 18-22° over the terraces themselves. On this terrace, as with the one below it, contour lines run diagonally from the edge of the terrace to the slope or wall behind it. This means the terrace surface slopes in a different direction to the mountain slope it’s created from, which runs perpendicular to the line of the terrace itself.
This is ingenious. It makes for a reasonably gradual slope along the line of the terrace and directly across it (see image below), so rainwater run-off is not lost as rapidly as it would otherwise be. Flowing diagonally, it spreads and spends much longer crossing the surface of the terrace, but still drains effectively without channeling or eroding terrace walls. Surprisingly, the average annual rainfall here – 1040mm – is slightly more than where I used to live in the very wet Scottish Borders, but just about all of it falls in the winter months, so effective drainage is no less critically important as water retention is for the summer months.
The existing fruit tree plantings however had been made following the line of the terrace, so the trick with the swales would be to space them in such a way as to leave no tree without a swale close enough behind it to irrigate it.
Since these swales won’t be fed by ponds like the terrace below, the idea is to fill them by hosepipe every 7-10 days through the summer drought, decreasing the frequency of irrigation as the soil improves year on year. To begin with, plantings will be limited to mulched swale berms so as not to place too much demand on water supplies through the summer, especially as the winter rains failed during the winter of 2014-15.
We had worn a well-compacted pathway along the middle of the terrace simply by walking along it frequently, so the first task was to create a new pathway and decompact as much as possible of the old. The narrowest part of the terrace is simply not wide enough to have room for both swale and pathway between the trees, so the logical solution was to make the swale itself into a path. With the contour lines running diagonally, this would result in the path meeting the back slope before it reached half way along the terrace. This turned out to be less of a problem than it first seemed – it was simple enough to end the swale at the back slope, then continue the path along the back of the terrace.
A small amount of compost was spread along the swale berms before planting and mulching.